As concerns for the environment grow, biodegradable polymers are becoming of increasing commercial interest as a substitute for conventional and less degradable synthetic polymers such polyolefins and polyurethanes. The use of biodegradable polymers in areas of packaging and textiles is therefore steadily growing. Consequently, much research is directed to the synthesis of these biodegradable materials. Among the various biodegradable polymers, polylactide (PLA) is one of the most commonly used and studied.
PLA is an aliphatic polyester based on lactic acid, the latter being obtained by fermentation of sugars and/or starch. PLA is therefore derived from renewable vegetables and is biodegradable by composting.
PLA is prepared from lactide by ring-opening polymerization. Lactide which is a dimeric cyclic ester of lactic acid can in turn be formed by dehydrating aqueous lactic acid to form a mixture of oligomers, which mixture is then depolymerised to form lactide.
A number of other processes exist for preparing lactide. In U.S. Pat. No. 5,900,491, a process is described in which lactide is synthesized from DL-methyl lactate material by addition of methyl orthoformate. The aim of said process is to provide high-purity lactide containing small amounts of impurities such as water and an acid component. Further, in U.S. Pat. No. 4,835,293, a process is disclosed for preparing highly pure cyclic esters such as lactide by heating a polymer of the corresponding alpha-hydroxy acid or its ester or a copolymer of the alpha-hydroxy acid or its ester and a thermally stable polyether core in the presence of an inert gas with the cyclic ester being carried from this reaction with the inert gas to a solvent system.
The lactide may as such have one of three types of optical activity depending whether it consists of two L-lactic acid molecules, two D-lactic acid molecules or an L-lactic acid molecule and a D-lactic acid molecule combined to form the dimer. These three types of dimers are known as L-lactide, D-lactide and meso-lactide respectively.
In order to prepare PLA of a high quality it is very important to use lactide of a high purity. The lactide needs therefore to be purified to a high extent before ring-opening polymerization may take place to prepare PLA of a high quality. In practice this means that the lactide needs to be free from water and impurities such as lactic acid, oligomers of lactic acid, catalyst, and light and heavy colour impurities as for instance sugars, nutrients, proteins and amino acids.
A variety of lactide purification processes are known that usually comprise one or more integrated distillation, condensation and crystallization steps. It is known in the art that crystallization processes or methods may be typically carried out in multiple stages such as falling film crystallization, suspension melt crystallization or static crystallization stages. U.S. Pat. No. 5,504,247 discloses such a crystallization method for the purification of acrylic acid. Similarly purification apparatuses may comprise different types of crystallization sub-units such as falling film crystallization, suspension melt crystallization or static crystallization sub-units. Additional information on crystallizers and their operation is disclosed in Handbook of Industrial Crystallization, 2nd Edition, by Allan S. Myerson, published Jan. 9, 2002 by Butterworth-Heinemann, Woburn, Mass. ISBN: 978-0750670128 and Crystallization Technology Handbook, 2nd Edition, edited by A. Mersmann, published 2001 by Marcel Dekker, Basel, ISBN: 0-8247-0528-9.
In U.S. Pat. No. 5,521,278, an integrated process for the manufacture of lactide is described in which aqueous lactic acid is first dehydrated in at least two stages to effect condensation polymerization of the lactic acid and the formation of oligomers. The oligomers formed are thermally cracked in the presence of a depolymerisation catalyst to form lactide vapour. The lactide vapour is then condensed and the condensate is vacuum fractionally distilled whereby lactic acid, water and lactide are removed as vapour overhead, whereas concentrated lactide is removed as a liquid side stream and heavy ends are removed as molten liquid. The liquid side stream of concentrated lactide is then subjected to melt crystallization to separate purified lactide having an acidity potential less than 6 meq/kg of lactide from a residual lactide having an acidity potential of at least 30 meq/kg.
In U.S. Pat. No. 5,357,034, a lactide polymerization process is described which comprises the steps of: (i) dehydrating aqueous lactic acid and polymerizing lactic acid by condensation to; (ii) heating and reacting an equilibrium mixture comprising mainly polylactic acid and small amounts of water, lactic acid, lactide and linear oligomers of lactic acid, and depolymerising polylactic acid to form lactide; (iii) distilling off a vapour phase comprising mainly lactide, lactic acid, linear oligomers of lactic acid and water; (iv) further fractionating in a separate unit operation the distillate obtained in the step (iii) into crude lactide and a distillate phase, comprising in addition to lactide, water, lactic acid, and linear oligomers, which is optionally recycled to step (i); and (v) purifying the crude lactide as obtained in step (iv) by melt crystallization; and (vi) bulk or solution polymerizing the purified lactide.
These known processes leave, however, room for improvement in terms of purity of the lactide obtained and condensation and crystallization efficiency.
It is therefore object of the present invention to provide a process for purification of a cyclic ester of an alpha-hydroxycarboxylic acid in which in particular a lactide of a higher purity and lighter colour is obtained and at the same time a more efficient condensation and melt crystallization is realized.